The industrial success of facing particleboard with decorative materials for use in walls, furniture and cabinets depends largely on achieving high levels of bond strength at relatively low cost. One widely used technique utilizes urea-formaldehyde (UF) resins to bond low weight decorative paper to various fiberboard or paperboard substrates. UF resins, however, are not generally used to bond high weight decorative paper, such as paper whose weight exceeds 60 grams/square meter, because of marginal to poor adhesive properties. It is possible to achieve desirable adhesive properties from UF resin adhesives for bonding high weight paper by use of high solids UF resin compositions. However, conventional high solids compositions utilizing UF resins are undesirable for a number of reasons including the high free formaldehyde content, poor shelf storage, poor tack, poor wetting characteristics, and poor clean up capability, among others.
Polyvinyl acetate (PVA) adhesives often are used as an alternative to UF to bond paper to various substrates, including lignocellulosic substrates. PVA adhesives have an advantage of fast cure speeds under marginal cure conditions but are also more expensive and frequently exhibit poor performance properties relative to those of UF adhesives. PVA adhesives can cause fiber swelling with subsequent undesirable protrusion of the fibers through the paper face. In addition, PVA adhesives exhibit little water resistance.
Two techniques are known in the art to make a UF resin which has a very low free formaldehyde content. One such technique calls for making a conventional UF resin of a low formaldehyde/urea (F/U) mol ratio, i.e., 1.2/1 to 1.5/1. A second technique utilizes an ammoniated UF resin. In the former technique, UF resins possessing low tack, poor stability, poor clean up characteristics and poor reactivity often result. In the latter technique, resins of low formaldehyde content can be achieved while maintaining desirable clean up and tack properties of the resins with high F/U mol ratios. Unfortunately, stripping the resin of solvent to achieve high solid levels often sacrifices storage capability and excessively high viscosities result. To counter this adverse result while maintaining high solids levels, the extent of the condensation reaction can be limited. This, however, results in a marked reduction in reactivity of the resin.
Therefore, it is desirable to have a high solids, shelf stable, low free formaldehyde, UF resin adhesive for bonding paper to substrates.